(1) Field of the Invention
The present invention relates to lithium-manganese-based composite oxides containing titanium and nickel, which are useful as positive electrode materials for next-generation, low-cost lithium-ion batteries, a method for preparing the same, and uses thereof.
(2) Description of the Related Art
The majority of secondary batteries presently mounted in portable equipment such as cellular phones, notebook computers, etc., in Japan are lithium-ion batteries. Lithium-ion batteries are also expected to become practical as large batteries for use in electric vehicles, electric load leveling systems, etc., and are increasing in importance.
Lithium-ion batteries of today use a lithium cobalt oxide (LiCoO2) as a typical positive electrode material, and a carbon material such as graphite as a negative electrode material.
In such a lithium-ion battery, the number of lithium ions that are reversibly deintercalated (i.e., charging) and intercalated (i.e., discharging) into the positive electrode determines the battery capacity, and the voltages during deintercalation and intercalation determine the battery operating voltage. The positive electrode material LiCoO2 is hence an important material for battery constitution, which is associated with battery performance. Demand for lithium cobalt oxide, therefore, is expected to grow further with the increasing range of applications and increasing size of lithium-ion batteries.
Lithium cobalt oxide, however, contains a large amount of cobalt, which is a rare metal, thus being a cause of the high material costs of lithium-ion batteries. Further, considering the fact that about 20% of cobalt resources are presently used in the battery industry, it seems to be difficult to meet the increasing demand with only positive electrode materials made of LiCoO2.
Lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4) and the like have been reported as positive electrode materials that are inexpensive and less limited as natural resources, and some of these materials are in practical use as alternative materials. With lithium nickel oxide, however, battery safety decreases during charging, and with lithium manganese oxide, trivalent manganese dissolves into the electrolyte during charging/discharging at high temperatures (about 60° C.), causing significant deterioration in battery performance. For these reasons, the use of these materials as alternatives has not progressed much.
Composite oxides such as LiNi0.5Mn0.5O2, LiNi0.5Ti0.5O2, LiNi0.45Mn0.45Ti0.10O2 and the like have been proposed as low-cost positive electrode materials that can substitute the aforementioned materials (see Non-Patent Documents 1 to 3 shown below). These positive electrode materials, however, contain the relatively expensive element, nickel, in an amount of 45% or more based on the total amount of transition metals, and therefore are not inexpensive enough. Moreover, none of the positive electrode materials reported in these documents exceeds an initial discharge capacity of 200 mAh/g.
Although Non-Patent Document 3 has reported that the incorporation of Ti into LiNi0.5Mn0.5O2 improves charge/discharge reversibility, it does not teach that the charge/discharge capacity can be increased by the presence of Ti. Moreover, the resulting Li2TiO3 is electrochemically inactive, and causes the theoretical capacity to decrease, thus limiting the amount of Ti to only 5% based on the total amount of transition metals (LiNi0.475Mn0.475Ti0.05O2).
As described above, various reports have been made on positive electrode materials that can substitute lithium-cobalt-based positive electrode materials; however, for further improvements in charge/discharge characteristics, the optimization of the chemical composition, preparation conditions, etc., of positive electrode materials is desired.
Non-Patent Document 1: T. Ohzuku and Y. Makimura, Chemistry Letters, 30[8], 744-745 (2001)
Non-Patent Document 2: M. Tsuda et al., J. Power Sources, 144, 183-190 (2005)
Non-Patent Document 3: J. S. Kim et al., Electrochemistry Communications 4, 205-209 (2002)